In the field of ventilation airflow control is essential and many airflow control devices/dampers are known. A very simple and cost efficient type is the so-called blade damper. This damper consists of a rotatable blade arranged inside an outer tube, and which blade can be rotated around a rod fitted in the outer tube. Preferably the blade comprises a sealing around its periphery to prevent leakage in the closed position. By rotating the blade, more or less of the airflow passage is covered by the blade and thereby it is possible to control or stop the airflow passing through the tube. Unfortunately this damper type causes a lot of turbulence of the airflow and thereby a lot of noise which are unwanted characteristics and a problem within ventilation.
Another well-known damper is the so-called IRIS-damper, which changes the size of the airflow passage via an iris-like radial movement of a number of thin steel sheet blades. This is a good solution for controlling the airflow, but unfortunately also a very noisy solution with a complex and cost driving design.
A recent solution is presented in EP 2 492 606 A1, which is an airflow adjustment device for arrangement in an air ventilation arrangement, and which comprises an outer tube, an inner tube arranged inside the outer tube and rotationally and axially moveable relative to the outer tube. Further the device comprises a twist tube with an axial opening through which air is adapted to flow. The twist tube is fixed relative to the outer tube with one end and attached to the inner tune at the second end. The inner tube is adapted to be rotated and displaced relative to the outer tube and when the inner tube is rotated the twist tube is twisted and the size of the axial opening is changed. This device is far better regarding noise compared to the previous presented devices, but it has a complicated design and is therefore expensive to manufacture. For all the above described prior art solutions a consequence of increased throttling, i.e. the closing of the damper, is increased turbulence and thereby increased noise.
Because of the above described shortcomings of existing dampers there is a need for a simple and cost-efficient air control device with better characteristics regarding noise and airflow control, preferably over a wide range of airflows for the specific size of the airflow control device.